Experimental Design - Comments, Flames Welcome

[Michael Rothacher]

Circlotron:

Super-Symmetry is the name Nelson Pass has ascribed to his U.S. Patent # 5376899. If you have a look at the patent, in figure 9 you'll see an op-amp style rendering of the invention. There's also an article on Super-Symmetry posted on the Pass Labs website. If you have an interest in balanced topologies, there was an excellent article in TAA 1/91 by Erno Borbely titled "Balanced Audio Amplifiers". I haven't yet tested the experimental design. I've made a number of changes to the schematic and I will post my changes and findings as soon as I can. I just completed the floating power supplies, so I'm hoping to test this weekend. I have tested the circlotron output stage on its own, however, and it appears to perform well, in fact it might make an interesting unity-gain power buffer albeit AC-coupled at the inputs.

Thanks

Mike

[Circlotron]

Looking forward to seeing your schematic. This is the extent of my thinking at this point. I'm sure there are a few things with this cct that others would do differently . The first set of opamps have their supply rails referenced to the same rails as the second so they too are bootstrapped. I have put rc's allover the place to damp things down a bit; I'm quite sure it would oscillate all over the place if I didn't. The idea is to fire it up and then reduce these rc's to a useable point. The bias is set by the 100mV forcing 100mA therefore 100mV across the source resistors. When the fet is on it's non-working cycle it will not turn fully off but instead have the constant 100mA through it. I think I will have to fiddle some of the biasing rc time constants so it won't try and follow the audio too much on the working half cycle of each fet.

GP.

[jan.didden]

To MROTHACHER:

On the patent side, the earliest reference to this type of circuit I found is from Philips. They had an early 60's implementation with tubes, and called it PPP for parallel push-pull, as the output tubes are paralleled for AC (and in series for DC). I built it at the time with 2 x 807's.

I noticed you used separate supplies for the output stage and the pre-stage. I think you can use the output stage (floating) supply to power the input stage as well. Note that the output supplies are offset from ground by half the output voltage. So, powering the input stage from this will give it a supply that tracks the output voltage to some extend if you chose the polarity correctly. That may even be beneficial for the linearity of the thing, since it decreases common mode swing from the input stage's point of view. Are you simulating this circuit? I would be interested how that would work out.

janneman

[Michael Rothacher]

Circlotron:

I should have mentioned that according to the X series manuals, Super-Symmetry cannot be done with op amps. Figure 9 simply illustrates the feedback and cross-coupling of the gain blocks.

janneman:

Using the floating supplies to power the input differential would be very interesting, but exactly how to carry it out eludes me at the moment.

Thanks

Mike

[Nelson Pass]

You could explore creation of an X circuit with
a pair of inverting "current feedback" type chips,
if such a thing exists. These would have say a
gate for the negative input and a source for the
positive input. More to the point, you would have
a power inverter on the output stage of each.

[Brett]

Hi Nelson,

Hope you had a great holiday.

The X-Zen idea has me intrigued. Is this something you were going to do some articles on later?

Regards

[Michael Rothacher]

The prototype is complete. I've made a number of changes to the schematic, which I'll post later tonight. The bias is (reasonably?) stable once the heatsink is warm. A condition which may take 20-30 minutes to reach. I'll also post my initial findings and several questions (of course questions!) as well.

As currently implemented, the circuit has increased gain at LF which I can't explain and I'm quickly exhausting my brief electronics education!

Thanks

Mike

[Nelson Pass]

The Xzen will be part of the Zen Variations series,
and I have made several different versions.

It is also my intention to release some commercial
product by the end of the year or so.

In the meantime I have no objection to you guys
working it up on your own.

[Petter]

So, when building an XZen -- basically the X1000 input stage with low voltages and high current -- it would probably be opportune to use op-amps to control the current sources to reduce power consumption (each volt dropped across sense counts at multiple amps quiescent current). Such a scheme would also enhance stability, but if I know NP right, he would do it with a small Zetex BJT ...

I suspect these sources can be lifted from the Aleph low side.


Petter

[Nelson Pass]

op amps. ugh.